A vaccine for feline leukemia virus (FeLV) will be developed by inducing specific anti-idiotype (anti-id) responses in cats against antibodies directed to FeLV. Vaccines for retroviruses have proven elusive, for generally inactivated viruses and sub-unit vaccines are poorly immunogenic; consequently a successful approach for one retrovirus is likely to be relevant for others (hopefully including the human T-cell leukemia viruses). FeLV is an infectious retrovirus and transiently infects about 50% of all pet cats by 4 years of age; about 10% of these become persistently infected, immunosuppressed and usually develop fatal non-regenerative or malignant hematologic diseases within 3 years of infection. Monoclonal antibodies (MAbS) that neutralize FeLV infectivity, and which are directed to either the conserved regions of FeLV gp70 of subgroups A,B and C, or are directed to the hypervariable regions which are subgroup specific, will be used to raise anti-idiotype antibodies. MAbS alone, and monoclonal anti-ids will be compared with rabbit polyclonal anti-ids (affinity purified), and each of the 3 types of antibodies will be used to immuize SPF kittens. The resulting anti-FeLV responses in cats will be dissected and analyzed and following immunization, recipients will be challenged with infectious FeLV in order to evaluate the protective efficacy of the immunizations. As certain anti-id antibodies resemble FeLV epitopes, these MAbs will bind to FeLV receptors present on a small sub-population of cat T-cells. The ability of such antibodies to bind to FeLV susceptible cat T-cells will be investigated with a view to specifically eliminating this population of T-cells without damaging the remaining T-cell repertoire.